Sintered silicon carbide varistors



1970 TAKESH! MASUYAMA ET AL 3,489,

SINTERED SILICON CARBIDE VARISTORS Filed Aug. 30, 1968 %A m m A MMm o 5wMS Tm W 2 w M T N Th R U :13 C m 4 w ATTORNE E United States PatentOfifice 3,489,532 Patented Jan. 13, 1970 US. Cl. 29-182.8 5 ClaimsABSTRACT OF THE DISCLOSURE A silicon carbide varistor having a lowvaristor voltage C and a high n-value which does not decrease withincrease in current flow is made by mixing fine particles of siliconcarbide with water, ceramic binder and pulverulent copper carbonate, andthen drying and firing the pressed body at 1080 C. to 1400 in an inertgas or in hydrogen, the proportions of ingredients being such that thefinal product contains 5 to 9 weight parts of silicon carbide, l to 5weight parts of ceramic binder and 0.1 to 1 weight part of copper, thelatter being partly dispersed in said ceramic binder and partly dilTusedin the surface layer of said silicon carbide particles. Such varistor isuseful in electronic equipment such as transistor radio and transistorTV. Graphite may be added.

This invention relates to silicon carbide varistors having non-ohmicresistance and to a method for making the same.

Silicon carbide varistors are ordinarily manufactured by mixing fineparticles of silicon carbide with water and ceramic binder, pressing themixture in a mold to the desired shape, and then drying and firing thepressed body. The electrical characteristics of such a varistor areexpressed by the relation:

where V is the voltage across the varistor, I is the current flowingthrough the varistor, C is a constant which is named convenientlyvaristor voltage and is equivalent to a voltage at a given current, andn is a numerical value greater than 1. The value of n is calculated bythe following equation:

where V and V are the voltages at given currents I and I respectively.

The desired value of C depends upon the particular use to which thevaristor is to be put. It is ordinarily desirable that the value of n beas large as possible since this exponent determines the degree to whichthe varistor departs from ohmic characteristics.

Recent progress in electronic equipment such as transistor TV hasrequired a varistor having a varistor voltage as low as C =30 v. and ann-value as high as possible. The notation C =30 v. means that a currentof 10 ma. produces a voltage of 30 v. across the tWo ends of thevaristor. Such requirement has been partly satisfied by a siliconcarbide varistor which includes graphite powder. However, it has beendifficult for the conventional silicon carbide varistor to achieve a lowvaristor voltage C and a high n-value. In addition, the n-value of aconventional silicon carbide varistor is apt to decrease with anincrease in the current flowing through the varistor.

An object of the present invention is to provide a silicon carbidevaristor having a low varistor voltage C and a high n-value.

A further object of the present invention is to provide a siliconcarbide varistor having a high n-value which does not decrease with anincrease in the current flowing through the varistor.

A further object of the present invention is to provide a method formaking a varistor characterized by a low varistor voltage and a highn-value which does not decrease with an increase in the current flowingthrough the varistor.

The objects are achieved according to the present invention,presently-preferred embodiments of which are set forth in the followingdetailed description, with reference to the accompanying drawings, ofwhich:

FIG. 1 shows a cross-sectional view of a varistor according to thepresent invention.

FIG. 2 shows an enlarged cross-section of a varistor element accordingto the present invention.

FIG. 3 shows a graphical illustration of a comparison of I-n curvesbetween a conventional varistor and a varistor according to the presentinvention.

Before proceeding with a detailed description of the nature of avaristor embodying the invention, the construction of a form of avaristor will be described with reference to the drawing of FIG. 1,wherein 10 indicates generally a varistor having, as its active element,a sintered body 1 composed of a composition contemplated by the presentinvention.

The sintered body 1 is provided on two opposite surfaces with electrodes2 and 3. The sintered body 1 has any suitable shape, for example, disc,square, plate or bar in a cylinder form. Wire leads 5 and 6 are attachedconductively to the electrodes 2 and 3, respectively, by a connectionmeans 4 such as solder or the like. The relationship of these parts isper se conventional.

The sintered body 1 comprises 5 to 9 weight parts of silicon carbideparticles, 0.1 to 1 weight part of copper and 1 to 5 weight parts ofceramic binder which bonds said silicon carbide particles together. Saidceramic binder is composed of any ceramic material inert to siliconcarbide particles at high temperature in non-oxidizing ambient, such asclay, pottery stone, refractory oxide and their combinations. A highamount of ceramic binder results in a high-electrical resistivity of thevaristors and a low amount thereof produces poorly sintered varistors.Desirable proportions of the ceramic binder to silicon carbide are 1 to5 weight parts of ceramic binder to 5 to 9 weight parts of siliconcarbide particles.

Referring to FIG. 2, silicon carbide particles 11 are bonded together bymeans of ceramic hinder 12 surround ing said silicon carbide particles11. Finely divided copper powder 13 is dispersed in said ceramic binder12. The surface layer 14 of said silicon carbide particles 11 has copperatoms difliused therein.

According to the present invention, a varistor element having such astructure shows a low varistor voltage C and a high n-value which doesnot decrease with an increase in the current flowing through thevaristor element.

Referring to FIG. 3, curve 1 is an I-n curve of a varistor comprisingcopper partly dispersed in the ceramic binder and partly diffused intothe surface layer of silicon carbide particles and curve 2 is an I-ncurve of a conventional varistor comprising no copper. For easycomparison, the two varistors are similar to each other in the weightproportion of silicon carbide particles to the ceramic binder. Theweight proportion is 5.0 weight parts of silicon carbide particles to4.0 weight parts of the ceramic binder. The varistor according to thepresent invention includes 0.5 weight parts of copper. The two varistorvoltages C are V =24 and V =80 for the novel varistor and theconventional varistor, respectively. It will be readily understood thatthe novel varistor having the structure of FIG. 2 has the low varistorvoltage and the high n-value which does not decrease with increase inthe current.

For convenience, a current coefficient of n-value will be defined aswherein n and n are n-values 1 ma. and 100 ma. of current respectively.

The thickness of said surface layer 14 varies with heating temperatureand heating time. It is not easy to measure the exact thickness of saidsurface layer 14. When the sintered body is immersed in HF solution, theSiC particles 11 do not dissolve in the HF solution but the ceramicbinder 12 and dispersed copper 13 dissolve into the HF solution. Achemical analysis of SiC particles separated from the ceramic binderindicates that copper diffuses into the SiC particles. There is adifference in the n-value and varistor voltage C between aggregate oforiginal SiC particles and that of the SiC particles separated from theceramic binder. Such difference further supports the diffusion of copperinto the SiC particles. When the SiC particles separated from theceramic binder are heated at 1000 C. in air, they are covered at thesurface thereof with SiO film formed by partial oxidation of SiC. TheSiO film dissolves into the HF solution. A repetition of air-heating andHF treatment removes the surfaces layer of the SiC particles. Theremoval of the surface layer of SiC particles which are separated fromthe ceramic matrix causes the SiC particles to have an n-value and avaristor-voltage C similar to those of the original SiC particles.

According to the present invention, an addition of graphite powdersdispersed in the ceramic binder causes the varistor element to have alower varistor voltage C without degrading the n-value in associationwith the copper dispersed in the ceramic matrix and diffused in thesurface layer. Operable particle size of said graphite particles rangesfrom 0.1;1. to 50,. Operable weight proportion of said graphite powderto said ceramic matrix is 4 to 14 wt. percent of graphite powder to 86to 96 wt. percent of ceramic matrix in accordance with the presentinvention.

Operable particle size of said silicon carbide particles ranges from 30to 200 Any available silicon carbide particles can be used as thestarting raw materials.

The best results are obtained with the varistor element comprising 6 to8 weight parts of silicon carbide particles, 2 to 4 weight parts ofceramic matrix and 0.1 to 0.5 weight part of copper, part of whichdisperses in said ceramic matrix and another part of which diffuses inthe surface layer of said silicon carbide particles. It is advantageousfor achievement of lower varistor voltage 1 4 C that said ceramic matrixhas 4 to 14 wt. percent of graphite powder dispersed therein.

The varistor element having a structure shown in FIG. 2 is prepared bypressing a mixture of silicon carbide particles, ceramic binder, copperpowder and, if necessary, graphite powder in a weight proportiondescribed above, into desired shape; heating the pressed body for 30minutes to 2 hours in non-oxidizing atmosphere such as nitrogen, argon,helium or hydrogen at a temperature higher than the melting point ofcopper, i.e. 1080 C. A heating temperature higher than 1080 C. isnecessary to efiFect the surface layer of silicon carbide particleshaving copper diffused therein as shown in FIG. 2. The upper limit ofthe heating temperature is1400 C Qbe'ca'use a heating temperature higherthan 1400 Cjre'sults in a low C varistor but in a poor n-value.

The starting copper powders have aparticle size ranging preferably from1 to 20 The starting copper powders can be replaced with any coppercompounds which are converted into copper when heated together with theSiC particles in a nonoxidizing atmosphere at a temperature higher than1080 C. Operable copper compounds are copper oxide (Cu O, and CuO),copper hydroxide [Cu(OH) and copper carbonate (CuCO in a finely dividedpowder form of 1 to 20,41.

According to the present invention, a varistor element heated in aninert gas such as nitrogen, argon or helium gas having an oxygen partialpressure of 1 to 20 mm. Hg produces a varistor voltage C lower than thatof a varistor element heated in hydrogen or pure nitrogen. The n-valuein the inert gas having an oxygen partial pressure of 1 to 20 mm. Hg isessentially the same as that in hydrogen or pure nitrogen.

Presently preferred illustrative embodiments of the invention aredescribed in the following examples.

EXAMPLE 1 Mixtures of the composition listed in Table 1 are Well mixedin a wet ball mill. The ceramic binder used is composed of crushed clay,crushed pottery stone and a small amount of flux. The total chemicalingredients in the binder are as follows: 72% SiO 18% A1 0 0.50% Fe O1.0% CaO, 1.5% Mgo, 1.5% B 0 1.0%Na O and 1.0% K 0, with 3.5% ignitionloss by weight. The SiC particles are in a commercially available gradeand include, as an impurity, 0.2 wt. percent of A1, 0.1 wt. percent ofFe and 0.02 wt. percent of nitrogen. The average grain size of thecopper powder and Cu O is about 10 and 211., respectively. The mixturesare dried and admixed with a small amount of water so as to be Wellcompacted. The mixtures are pressed at 500 kg./cminto discs. The presseddiscs are heated in nitrogen gas having an oxygen partial pressure ofabout 1 mm. Hg at various temperatures for 1 hour as shown in Table 1.

The sintered discs are of a dimension of about 15 mm. in diameter andabout 0.8 mm. thick. The sintered discs are provided on the oppositesurfaces with zinc electrodes by the vacuum evaporation method. Aftercompletion by means of two lead wires, the varistors are tested for then-value, varistor votlage C and the current dependence of I. The resultsare shown in Table 1.

EXAMPLE 2 Mixtures of compositions listed in Table 2 are prepared afterthe manner described in Example 1 and pressed at 500 kg./crn. intodiscs. The pressed discs are heated in H at various temperatures for 1hour as shown in Table 2. The sintered discs are of'a dimension of about15 mm. in diameter and about 0.8 mm. thick. The sintered discs areprovided on the opposite surfaces with zinc electrodes by the vacuumevaporation method. After completion by addition of two lead wires,thevaristors are tested for the n-value, varistor voltage C and the cur-.rent dependence of I. The results are shown in Table 2.

5 EXAMPLE 3 Mixtures of compositions listed in Table 3 are preparedafter the manner described in Example 1. Graphite powder used is of acommercially available grade. The

other ingredients of the mixtures are similar to those of 5 zincelectrodes by the vacuum evaporation method. After 10 completion byaddition of two lead wires, the varistors are tested for the n-value,varistor voltage C and the current dependence of I. The results areshown in Table 3.

6 EXAMPLE 4 Mixtures of a composition similar ot that of No. 3 of Table1 are pressed into discs in a way similar to that of Example 3. Thepressed discs are heated for 1 hour at various temperatures in nitrogengas having various oxygen partial pressures as shown in Table 4. Thesintered discs are provided on the opposite surfaces with zincelectrodes by the vacuum evaporation method. After completion byaddition of two lead wires, the varistors are tested for the n-value,varistor voltage. C and the current dependence of I. The results areshown in Table 4.

TABLE 1 Electric characteristics of resultant resistor Weight proportionHeating Current, tempercoeflicient Copper Ceramic SiO ature, of nvalue,powder C1120 binder particles C. C at 10 mA n percent Sample No.:

1 2. 9. 0 1,200 so 3. 2 40 2 5. 0 9. 0 1, 200 19 4. 0 3 1.0 5. 0 1, 20030 4. 8 3 4 4. 0 9. 0 1, 200 29 4. 8 2 5 4. O 5. 0 1, 200 25 4. 6 0 6.1.4. 0 5. 0 1, 150 30 4. 6 0 7 4. 0 5. 0 1,250 22 4. 6 0 8 4.0 5. 0 1, 30018 4. 5 0 9 1. 5. 0 9. 0 1,200 4.2 0 10 0. 1 1.0 5. 0 1, 200 30 4. 6 211 0.3 4.0 9.0 1,200 28 4.6 3 12 0.5 4.0 5. 0 1,200 24 4. 6 0 13 0.5 4.05. 0 l, 150 28 4. 6 1 14 0.5 4.0 5. 0 1, 250 22 4. 6 0 15 0. 5 4. 0 5. 01, 300 18 4. 5 0

TABLE 2 Electric characteristics of resultant resistor Weight proportionHeating Current tempercoefiicient Copper Ceramic SiC ature, of n-value,powder C1130 binder particles C. C at 10 mA 11 percent Sample No.:

TABLE 3 Electric characteristics of resultant resistor Weight proportionCurrent Heating coetficlenc Ceramic SiC temperoi n-value, Graphite C1110binder particles ature C. C at 10 mA n percent Sample No.:

TABLE '4 Electric characteristics bi I resultant resistor Oxygen Firing.Current partial tempercoeflicient pressure ature, of n-value, Sample No.(mm. Hg) C. O at 10 mA 11 percent What is claimed is:

1. A sintered varistor element consisting essentially of 5 to 9 parts byweight of silicon carbide particles, 0.1 to 1 part by weight of copperand 1 to 5 parts by weight of a ceramic binder.

2. A sintered varistor element as defined by claim 1, wherein saidcopper is partly dispersed in said ceramic matrix and partly dilfused inthe surface layer of said silicon carbide particles.

7.18 3. A sintered varisto-r element'as defined by claim 2, wherein saidceramiematrix has 4 to 14 wt. percent of graphite powderdispersedfth'e'rein.

4. A slntered' varlstor cons sting essentially of 6 168 parts by weightof siliconfcarbide particle's, 2 to 4 by I graphite powder dispersedtherein v References cited' UNITED STAT BS PATENTS 6/ 1940 2,364,108 12/1944 Swantzel 204 X 2,601,373 6/1952 Dienal 25'2-504 X 3,153,636 10/1964Shanta 252-504 X 3,205,465 9/1965 Lambertson 252-504 X 3,404,031 10/1968Clayton 252502 CARL D. QUARFORTH, Primary Examiner A. I. STEINER,Assistant Examiner ,U.S. c1. X.R.

29 1s2.s; 7s-204, 206; 252-504 Pirani 252504 X

